1. Field of the Invention
This invention relates to a liquid crystal composition and, more particularly, to a liquid crystal composition for use in an electro-optic cell which contains the liquid crystal composition interposed between two opposed electrode plates, which enables the realization of good color displays utilizing the guest-host effect of liquid crystal.
2. Description of the Prior Art
In general, displays using liquid crystal are classified into two groups: one group utilizing the electro-optic effect of the liquid crystal material per se; the other group utilizing the electro-optic effect resulting from the mutual action between liquid crystal material and other ingredients. Typical examples of the latter are compositions which are prepared by dissolving a dye, called a pleochroic dye, in a nematic, cholesteric or smectic liquid crystal. The pleochroic dyes are roughly categorized into two types. One type of dyes are those in which the direction of transition moment of visible light absorption is almost parallel to the long-axial direction of the molecule and which, when dissolved as a guest molecule in the above-described liquid crystal, has the property that the long axis of the dye molecule and the molecular axis of the liquid crystal are substantially oriented in the same direction. These dyes show a property referred to as parallel dichroism. The other type of dyes are those in which the direction of transition moment of visible light absorption is almost perpendicular to the long-axial direction of the molecule and which, when dissolved in the above-described liquid crystal as a guest, has the property that the long axis of the dye molecule and the molecular axis of the liquid crystal are substantially oriented in the same direction. These dyes are thus pleochroic dyes showing a property referred to as vertical dichroism. Of these two types, the present invention relates to a liquid crystal compositions containing the former type of dyes, i.e., pleochroic dyes showing parallel dichroism. The degree of orientation of the pleochroic dye dissolved in the liquid crystal material can be quantitatively indicated in terms of an order parameter S to be described hereinafter.
When a nematic or cholesteric liquid crystal containing the pleochroic dye is interposed between two opposed electrode plates and an electric potential is applied across the electrodes, a disturbing movement of the liquid crystal molecules is caused, or a molecular alignment oriented in the direction of electric field is formed, depending on the dielectric properties and flowability of the liquid crystal. Under such conditions, the pleochroic dye molecules also move together with the liquid crystal molecules, causing a change in the relative relation between the direction of the absorption transition moment of the pleochroic dye molecules and the direction of incident light. As a result, the liquid crystal display undergoes a change in its light absorption properties. This phenomenon is well known as the "guest-host effect", and an electrically controllable color display device can be constructed utilizing this effect (see "Guest-Host Interaction in Nematic Liquid Crystals: A New ElectroOptic Effect", G. H. Heilmeier and L. A. Zanoni, Applied Physics Letters, vol. 13, p. 91 (1968)).
For example, when a nematic liquid crystal containing a pleochroic dye showing a parallel dichroism is interposed between two transparent electrode plates having been subjected to homogeneous orientation treatment and facing the parallel with respect to each other, the liquid crystal molecules from a homogeneous alignment wherein the long axes of the molecules are aligned in a direction parallel with the electrode planes. In this situation, the molecules of pleochroic dye dissolved in the liquid crystal are also aligned so that their long axes are also parallel with the electrode planes. When white light having travelling in a direction perpendicular to the electrode plane is transmitted through the guest-host material in the above-described alignment, the electric vector of incident white light is parallel with the long axes of the pleochroic dye molecules, and hence a specific wavelength region of the incident light is strongly absorbed by the pleochroic dye guest material. As a result, the guest-host material appears colored. (See FIG. 1.) When an electric field is applied to the liquid crystal material in the above-described alignment, if it has a positive dielectric anisotropy, the host liquid crystal molecules and the guest pleochroic dye molecules take on a homeotropic alignment wherein the axes of the molecules are perpendicularly aligned with respect to the electrode planes, due to the positive dielectric anisotropy of the host liquid crystal (see FIG. 2). In this situation, the electric vector of the incident white light is perpendicular to the axes of the pleochroic dye molecules, and hence the incident light is only scarcely absorbed by the pleochroic dye molecules, and the guest-host material appears transparent. Accordingly, the difference in color between the colored state and the transparent state enables the formation of optical displays.
Furthermore, when a pleochroic dye showing parallel dichroism is dissolved in a host material such as a nematic liquid crystal having a spiral structure (which may be caused by the addition of an optically active material) or a cholesteric liquid crystal having an inherent spiral structure, the pleochroic dye molecules are aligned in the spiral structure similarly with the host material. (See FIG. 3.) When light travels parallel to the helical axes of the guest-host material, the guest-host material transmits the incident white light according to two normal modes, each of which forms right-handed or left-handed elliptical polarization. The direction of the electric vector indicating each of these modes is closely related to the long axes of the guest molecules; a specific wavelength region of the incident light is absorbed by the guest material, and, as a result, the guest-host material takes on a colored state. Then, when an electric field is applied thereto in a direction parallel to the helical axes of the guest-host material, the helical structure of the guest-host material is unwound, to produce a nematic state wherein the long axes of the liquid crystal molecules and the dye molecules are oriented in the direction of the electric field, as long as the dielectric anisotropy of the host liquid crystal is positive (see FIG. 4). In this alignment, the direction of absorption transition moment of the pleochroic dye molecules is perpendicular to the electric vector of incident white light, and hence the incident light is scarcely absorbed by the guest molecules. Thus, the guest-host material appears transparent. Such a color display method is described in detail, for example, in Japanese Patent Application (OPI) No. 127645/74 (The term "OPI" as used herein refers to a "published unexamined Japanese patent application".).
In the case of using a smectic liquid crystal as a host liquid crystal, display is also possible by employing a suitable cell structure and a driving method utilizing the difference between the colored state and the transparent state similarly with nematic and cholesteric liquid crystals. Examples using a smectic liquid crystal are described, for example, in "New electro-thermo-optic effect in a certain smectic liquid crystal with a pleochroic dye added", C. Tani and T. Ueno, Applied Physics Letters, Vol. 33, p. 275 (1978).
In order to obtain an excellent contrast between the "on state" and the "off state" of a liquid crystal display cell utilizing the above-described guest-host effect, the guest of pleochroic dye must have such properties that, in one state, it appears strongly colored and, in the other state, it appears nearly transparent and colorless. That is, in order to give a strong color, the absorption transition moment of the pleochroic dye must be parallel with respect to the electric vector of the incident white light, or perpendicular to the light-travelling direction. On the other hand, in order to provide a nearly transparent colorless state, the absorption transition moment of the pleochroic dye molecules must be perpendicular with respect to the electric vector of the incident white light, or parallel to the light-travelling direction.
However, the use liquid crystal molecules and dye molecules results in disordered thermal fluctuation in orientation with respect to the alignment, and hence it is impossible for the absorption transition moment to be completely perpendicular or parallel to the light-travelling direction. Therefore, the degree of order of the alignment of dye molecules in the liquid crystal in a specific direction exerts great influences on the contrast of the cell. The degree of orientation of dye molecules in the liquid crystal medium is usually indicated in terms of a numerical values called the order parameter. The order parameter S indicates the parallel degree of the absorption transition moment of the dye molecules with respect to the oriented direction of liquid crystal molecules (usually indicated as as vector called director), and is defined as follows: ##EQU1## wherein the term of cos.sup.2 .theta. is timewise averaged, and represents the angle at which the absorption transition moment of the pleochroic dye meets the oriented direction of the liquid crystal (director). Order parameter S of a pleochroic dye dissolved in liquid crystal can be determined according to the formula ##EQU2## wherein A.sub..parallel. and A.sub..perp. respectively represent absorbances of dye molecules for the light polarized in parallel or perpendicular direction with respect to the orientation direction of the host liquid crystal (director). Accordingly, by determining A.sub..parallel. and A.sub..perp. through measurement of the absorption spectrum, the order parameter S of a dye in a host liquid crystal can be obtained to thereby evaluate the orientation properties of the dye. A description of the method for measuring the order parameter of pleochroic dye dissolved in liquid crystal as a guest is provided in "Absorption and Pitch Relationships in Dichroic Guest-Host Liquid Crystal Systems", H. S. Cole, Jr. and S. Aftergut, Journal of Chemical Physics, Vol. 68, p. 896 (1978).
The order parameter S of a pleochroic dye dissolved in a host liquid crystal can theoretically take the values from -0.5 to 1. With pleochroic dyes showing parallel dichroism, the nearer the order parameter is to 1, the greater the degree of order of the dye alignment in the liquid crystal. Therefore, in the display cell using a pleochroic dye showing parallel dichroism, contrast of the cell can be improved by using a dye having an order parameter as near as possible to 1.
In an electro-optic cell which comprises a liquid crystal containing dissolved therein a pleochroic dye and being interposed between two opposed electrode plates which enables the formation of a color display based on the guesthost effect, the pleochroic dye must possess: (1) a high order parameter S in a host liquid crystal so as to provide a large contrast between the colored state and the colorless state; (2) a large molecular extinction coefficient, so as to realize a distinct color by adding the dye to a host liquid crystal material in a small amount; (3) excellent stability with respect to light, heat, water, and oxygen; (4) a solubility high enough for the liquid crystal to provide an optional concentration within a necessary range; and (5) no ion-dissociating properties that will exert detrimental influences on the electrical characteristics of the cell. Of these, the order parameter value S (1) usually changes to some extent depending upon the kind of host liquid crystal, dye concentration, temperature, etc. even if the same dye is used. As to solubility (4), the dye desirably possesses a sufficient solubility so that the necessary concentration can be attained.
The dye concentration is selected depending upon the thickness of the liquid crystal layer of the liquid crystal cell and upon the extinction coefficient of the dye molecules.
Of known dyes, there are an extremely few compounds which satisfy all the conditions of (1) to (5) described above and can be satisfactorily utilized for guest-host type liquid crystal displays. Merocyanine series, azo series, azomethine series, an anthraquinone series dyes have been described as pleochroic dyes for guest-host type liquid crystal display in Japanese Patent Application (OPI) Nos. 56386/75, 2885/77, 126033/78 and 71088/79, U.S. Pat. Nos. 4,122,027; 4,128,496, 4,128,497 and 4,137,193, British Pat. No. 1,459,046, etc. However, these dyes are considered to only partly satisfy the above-described conditions (1) to (5).
Yellow series dyes are useful not only as yellow color forming ingredients but necessary as ingredients for compositions having a color of orange, green, black, or the like.